Please visit the accompanying website: Life on Nu Phoenicis IV, the planet Furaha.
This blog is about speculative biology. Recurrent themes are biomechanics, the works of other world builders, and, of course, the planet Furaha.

Sunday, 8 March 2009

Centaurism II

A while ago I discussed the goumoun, a six-limbed animal. I thought that freeing a pair of limbs for another purpose definitely occurred more than once in evolution, but that this principle sadly had no name. So I took it upon myself to call it 'centaurism'. Do not get me wrong: I am not claiming eternal fame for this invention, nor do I think evolution biology will suffer at all if it does not spread beyond this page.

Just treat it as an intellectual game. The original centaurs in Greek mythology had the body of a horse, but where its neck and head should be you would find the torso, arms and head of a human. Ancient Greeks were notoriously vague about the evolutionary history of their mythological beings, so we know very little about protocentauroids. Nothing, in fact. Did they walk on six limbs? The Greeks were probably more inclined towards a form of creationism: 'Poof!', leading to 'here's one I made earlier'.

On Furaha, the ancestors of neocarnivores did walk on six limbs in the typical hexapod fashion. There is nothing wrong with walking on four limbs, as tetrapods on Earth show, and indeed on Furaha. There was therefore ample room for evolutionary experimentation with front limbs in hexapod Furahan animals. They accordingly evolved into clubs, spears, lances, hatches and even nets (in the Microraptoria). If you want to see a few examples, simply travel to the land page on the Furaha site and have a look. Here's one of them:

Copyright Gert van Dijk

But centaurism is not confined to the goumoun or to Furaha. In fact, there are quite a few examples on Earth. Let's start with animals that started with four legs: are there any that stopped walking on all fours and got up on their hind limbs? Definitely; there are ostriches and other ratites, kangaroos, predatory dinosaurs and people, to name the most obvious.

What do they all do with their freed front legs? Some animals use them for grasping purposes at some times but to walk with at other times. Examples are the giant panda, lots of primates, as well as the kangaroo. If kangaroos move very slowly, they move on five limbs: the hind legs swing forwards while the body rests on the front legs and the tail.Other animals do not appear to do very interesting things with their front legs, such as ostriches. I have seen a male ostrich flapping it to attract females, but that is about it. In fact, some Furahan analogues do exactly that, but, having freed not just two but four limbs, they look even odder (the Grec on the land page is a perfect example). The same probably goes for the front limbs of Tyrannosaurus. I know that its arms were less weak then they look, and am aware of theories saying the beast used its arms to help stand up from a lying position. But all in all these puny arms do not impress me at all. Faced with animals that do not do anything interesting with their freed front legs, perhaps the definition of centaurism should include a new non-locomotory purpose for the freed legs.

That thought reduces the list: primates are still in, as well as those predatory dinosaurs that used their front limbs to do something interesting with. That group goes under the name of Maniraptora. One of the most intriguing examples would be Therizinosaurus, here shown as seen in the BBC documentary 'The Giant Claw', a 'Walking with Dinosaurs special'. Not that much is known about the animal apart from its front legs with their huge claws, so the claws are to the right scale, but the rest is educated guesswork.

BBC Walking with Dinosaurs Specials: The Giant ClawClick to enlarge

There is no reason to limit the principle of centaurism to animals starting with four walking limbs. Crabs are decapods ('ten-leggers'), but walk on eight legs. The front pair, the claws, are modified legs. In fact, the modifications are not that extensive, as is shown on an excellent animation on the internet from the University of Alberta and explaining how crabs' claws evolved and work. Here is a non-moving fragment to entice you to go and visit the original.

Click to enlarge

There are many other arthropod examples. Here is another one: Amblypygi or whip scorpions are spiderlike, but no longer walk on eight legs. Instead, one pair has evolved into long whiplike sensory organs. The next two photographs are from 'Life in the Undergrowth', another BBC documentary. It has some excellent footage of these animals. The whips are the thin limbs between the pedipalps and the walking legs. If you need more, check up on Uropygids in Wikipedia.

BBC Life in the UndergrowthClick to enlarge

In conclusion centaurism is fairly common. The modified legs are front pairs in all cases. I have not found any examples of a hind pair of legs becoming centaurised. That is probably because of 'cephalisation': the front end of a body commonly has the most sensory organs, food goes in there, etc. Once you have cephalisation, any manipulatory limb is best placed at the front. But if anyone can think of a sound evolutionary way to free hind limbs from locomotion in an animal that is already thoroughly cephalised, AND that might fit on Furaha, I will consider making a sketch of one.

10 comments:

StevenH
said...

The best example I can think of for "centaurisation" would be the mantis. I really am surprised that it wasn't mentioned! (But I admit that it was my favorite beastie in grade school, so I am admittedly biased.)

As for rear limbs being centaurised, there is the possibility of both the earwig and centipede. Both have pincer-like limbs that could have been legs at some point in the past (I don't know enough entomology to be sure, though).

By the way, I really like your blog. Now that I know it exists, I am going to be visiting often!

You are absolutely right: I had thought of the praying mantis when I started thinking about the subject, but forgot it later. By the way, mantids were not the inspiration for Furahan neocarnivores; mantis shrimps were (stomatopods). A fine point, I admit.

Interesting thing about the earwig; I will see if I can find anything on it.

First of all awesome blog. As for rear legs being centaurised is that what you are trying to do with those arboreal primate things on the home page? It appears the front limbs are for locomotion and the lower two limbs are for handling food and other items of interest

You're commenting on a post almost a year old, did you realise that? As the author I receive notification of that, but I doubt anyone else does.

First of all, thanks again, this time for noting the new front page. Secondly, no, the middle and hind legs of these animals are generic limbs, in this animal mostly used to cling to tree trunks and stems. They use their jaws to handle food, which in their case largely boils down to killing it....

Heh, I comment on years-old blog posts all the time. They're repositories of knowledge after all.

After reading the post I was going to mention those arboreal creatures as well. But another possible example, wandering far away from the usual image of centaurs, would be flies. Their rear pair of wings has become a tiny sensor pair used in flight.

I wonder if you would mind if I tried to adapt this post into an article on the Speculative Evolution wiki?

I recall one book on evolution stating that generally speaking, evolution moves in the direction of either specializing limbs or reducing their number over time.

Daniel, commenting on a four-year old post is probably equal to a private conversation with the blog owner. I do agree that old posts are there for a purpose, but that purpose is probably a one-way communication from author to reader.

I am aware of 'Diptera'; their very name reveals that something happened to one pair of their wings.

You seem to like my old posts, which is nice. Nevertheless, I am a scientist by profession and prefer the rules that govern quotations: a single sentence or phrasing a sentence encapsulating a conclusion may be quoted followed by a reference; quoting larger sections raises eyebrows even when referenced, and quoting without referring to the source is simply wrong. If you write something quoting me as the source of 'centaurism' in this context that is proper. Whether 'adapting' my post is proper depends on what you mean by that. Copying is never acceptable.

Wasn't there a backlash against the Speculative Media Wiki on the Spec biology forum a while ago? I did not follow that discussion in detail, but am wary. What is your interest in that Wiki?

I suppose that’s true; at the moment this is a private conversation. But it’s being held in a public place and I tend to think that if I’ve found something, other people may well do the same. And I’m OK with conversations which occur over the span of a decade, they can still be useful.

Your response regarding the wiki surprises me a little, but of course that’s why I asked, isn’t it? :) My position agrees with yours when I am writing something as an individual, but to me wikis are understood to be a conglomeration of different voices (ideally some sort of consensus) so as long as it’s OK with the author, pasting in material and modifying it (or not, as appropriate) is fine. Of course, Wikipedia itself requests contributors paraphrase from sources, not copy and paste, which is definitely what I stick with and what I was intending regarding this post.

My interest in the Speculative Evolution wiki is easily explained, if a bit naïve. I’ve toyed around with ideas on how to ‘properly’ design aliens for years, saw Alien Planet a couple of years ago, but only a few days ago, seeing The Future is Wild, did I realize there was anything else out there. Since Wikia sites show up high in google results, it was one of the first things I ran into and it seemed as good a place as any to put my own thoughts on the matter. I’ll look into the backlash you mentioned though.

Daniel: I was simply too busy to respond earlier. thinking about the subject of a Wiki some more, I think my hesitation has two sources: the first is a fear that my hard-earned knowledge appears elsewhere without references, making it appear as common knowledge. That is the eventual fate of all discoveries, but I would prefer to have that happen to my work later rather than sooner. The second hesitation had to do with this particular Wiki, which gives the impression that it is based on fan fiction of just one project, 'The Future is Wild'. That is fine for fans of that project, but the setting is not one I personally feel at home with.

The wiki is trying to clean itself up, delete implausible fan organisms, label whether organisms are canon and where they come from, and actually write a series of articles on biology at large. It is still kind of a mess but far less of one than a few months ago.

How about an article not making any direct us of your post, paraphrasing a little bit of it but mainly using other examples and ideas, and also citing your post as the source? Would that be OK?